This invention relates to the protection of metal piles from corrosion in underwater and semi-underwater environments and more particularly to the protection of angular metal piles and especially H and I beam type piling from corrosion in such environments.
Metal piles have in recent years widely supplanted the use of wooden and concrete piles for the support of docks, wharves, piers, drilling platforms and other marine structures above the surface of bodies of water. Metal piles are initially stronger and easier to handle and drive into marine sediments than other types of piles. One very economical pile to use is the so-called H-beam or I-beam type pile which is nothing less than the conventional H or I-beam so familiar in other types of structural work. These structural shapes are readily obtainable and easily adaptable to use as piling type supports.
Unfortunately H-beam and I-beam type piling, while strong and rigid for its weight, has large surface areas which, being made of metal, and usually iron, are subject to oxidation and other corrosive attack when exposed to corrosive environments. Iron, as is well known, is not stable when subjected to the usual surface atmospheric conditions. Under such conditions, unprotected iron will oxidize to produce various oxides of iron which are more stable under surface conditions than the uncombined metal. Such oxidation or corrosion, as is well known, is accelerated beneath the surface of bodies of water, especially sea water. Corrosion of metal surfaces and especially iron surfaces is particularly severe in the so-called "splash zone." The splash zone is the zone near the surface of bodies of water, which is alternately exposed to water and air due both to the changing level of tides and the like, the breaking of waves, the spray from waves and various other turbulences coming in contact with metallic structures. Corrosion of untreated metal surfaces is particularly severe in this splash zone and frequently results in a very short useful life for metal structures located in this zone. Very frequently metal piles will be found to be severely corroded and reduced in area in the portions of piles exposed to the splash zone even when corrosion in other portions of the piles is not severe.
Various means have in the past been used to protect metallic members from corrosion under both atmospheric and underwater conditions. Such means have varied from the painting of the surfaces of structures to the provision of internal alloying elements which decrease the oxidation of the metal incorporated in the structures. Such means, which have been found quite suitable under surface or atmospheric conditions, have in general been found to be effective only to a minor degree in the splash zone.
One means of protection known for the prevention of corrosion of underwater metallic structures is the use of sacrificial anodes. Such anodes are mounted near the metal member to be protected, and a portion of the corrosion action is transferred to the more readily corroded anode. Sacrificial anodes are often satisfactory for completely submerged objects but they have little effect upon accelerated corrosion in the splash zone.
Another widely used means for the protection of pilings and other supports from corrosion is a jacket or layer of some noncorrodible or corrosion resistant material placed around the piling or support. it is often difficult to apply such non-corrodible coatings to the pile and they are frequently subject to damage by mechanical abrasion and the like which may destroy the seal with the underlying structure. Most such coatings, furthermore, for example, coatings of brush-on or spray-on paint or the like, are only practical for application prior to the time a pile is installed and are very difficult to repair or brush up once the pile is permanently installed under water. Naturally any thin coating such as a coating of paint is also very easily damaged by abrasion or physical impact of any type.
In recent years coatings composed of sheets of plastic have been applied to underwater piling by means of strapping or the like. Fairly rigid plastic sheets may be handled by divers underwater but are not easily applied to H or I beam type metal piles or other similar irregularly shaped piles.
Various types of forms or coatings for piles have been developed for the coating of piles or the like underwater. The following U.S. patents are representative of disclosures of such prior art coatings.
U.S. Pat. No. 1,013,758 issued Jan. 2, 1912 to Fox et al. discloses a form for the application of concrete to piling. The forms are secured around the pile by means of collars made of segmented sections 3 secured about the forms by means of banding straps 4.
U.S. Pat. No. 2,874,548 issued Feb. 24, 1959 to Drusbel et al. discloses a corrosion resistant plastic shield or sleeve 19 which contains a body of sealing material 27 which excludes water from the space 25 between the sleeve 19 and the metal pile 11. Drusbel states that, "among the materials suitable to fill the space 25 are heavy dielectric greases and asphalts which are sufficiently solid to remain fixed securely in place within the sleeve."
U.S. Pat. No. 3,321,924 issued May 30, 1967 to Liddell discloses the use of a plastic sheet wrapped about a wooden pile and secured with straps to exclude marine borer attack. The plastic sheets are applied in situ by a diver.
U.S. pat. No. 3,370,998 issued Feb. 27, 1968 to Wiswell discloses a corrosion resistant coating for metal piles including H-beam piles. The surface of the pile is first thoroughly cleaned in situ. A plastic resin such as an epoxy resin is then mixed with a curing agent and applied to a preferably porous backing member. Mixing and application to the backing member is preferably done in a mixing tray having rims of a height suitable to contain a layer of plastic of the thickness desired. The porous backing and plastic are then placed onto a metal form having the shape of the piling to be coated and the form is then held against the pile until the plastic cures. The form can either be left against the pile, or, if a suitable separating means such as a polyethylene sheet is inserted between the form and the plastic, the form can be removed after the plastic hardens. The entire assembly of foam plastic and backing member (which may be a fiberglass or other cloth sheet) is installed in place by divers and the assembly is secured in place by straps. In case of an H-beam the forms may be comprised of metal, plywood, masonite or other suitable material.
Such prior disclosures and practices, while effective to various extents to apply corrosion resisting coatings to piling in general, have not been notably successfully in applying corrosion resistant coatings to H or I-type metal piling or other irregularly shaped piling. It has in particular proven very difficult, and in many cases substantially impossible, to obtain a good interfit which is effective to exclude moisture between the inside of the outer plastic coating and the outside of the H or I-pile due to the inconvenient shape of such piling.